ADA MP-1 (original) preamp - noisy

randella

Hi All

Dug out my old ADA last night for a blast, and remembered a) how much I like the tone, and b) just how noisy it is.  I'm not expecting it to be modern whisper silent but there's a lot of 50Hz hum present, plus a much more irritating highish-frequency whine with a rhythmic (maybe 2Hz) oscillation in volume all of which to my ears sounds like a clock signal.

The noise is present on all preamp settings, regardless of output level.  In place of a power amp, I'm using it in the effects return of a Mesa Mini Rectifier and controlling the overall volume with the MP-1's output level knob.

It's an ancient unit I bought for £70 in the late nineties, off a tech who claims it'd been on tour with a thrash metal band called Dearly Beheaded.  The story made me chuckle anyway, and when I got it home pretty much every single preset was a thick, heavy distortion with the odd crystal-clean-chorus patch for good measure so maybe it was true.  Who knows.

Either way, it ain't pristine.

Anyone any ideas as to what and what shouldn't be replaced?  It's been worked hard and I've no doubt several components will be past end of life.

ICBM

The hum could be a ground loop - a lot of rack equipment is poorly designed for this given that it will certainly be connected to something else which is likely to be earthed. If so, DO NOT undo the earth in the mains plug - you either need a ground-lifted audio cable to connect it to the amp, or to have it modded for a proper ground lift.

I'm not sure what the whine is but it could be a clock frequency from the MIDI section leaking into the audio path.

randella

@ICBM Many thanks - am I right in thinking that cutting the connection between the jack and the outer screen in the amp end of the connecting cable would be a place to start?

I seem to remember when I got the unit it had a very short mains cable (inbuilt), probably where it was fitted into a touring rack.  I replaced this and now I think about it the earth was not connected, which I put back.

ICBM

randella said:

@ICBM Many thanks - am I right in thinking that cutting the connection between the jack and the outer screen in the amp end of the connecting cable would be a place to start?

Probably, although sometimes it needs a resistor in the break - usually around 10 or 100 ohms.

A better solution is to properly ground-lift the unit, although this can be difficult sometimes. A friend of mine has just started getting into old rack gear and has bought a lot of this sort of stuff, so I'm getting quite familiar with this problem . Of course in the old days everyone just used to undo the earth in the mains plugs...

randella

Ok, thanks for the help as ever @ICBM.  I'll try with the signal cable later on.  I'm using it with a multi-fx too, which it's earthed to by dint of the fact it's screwed into the same rack.  Back to basics then until I find the culprit!

Although it sounds great, you can see why this stuff went out of fashion...

randella

@ICBM A bit more investigation, if you or your friend are interested.

It looks as though there have been repairs made to the unit.  All the big electrolytics look to have been replaced (valve and power supply filters); the soldering's neat but obviously not original.  The two 12AX7s are kept on a daughter board and this has been modified - some shielded cable added at some point that carries the signal back to the mainboard.  I put in some new JJ's a while back which made the square root of sod all difference to either the tone or the noise floor.

All the 1/4" sockets on these preamps should be plastic PCB-mounded items.  Most of the ones on mine still are but have had metal washers and securing nuts added.  The front-panel input jack has been changed for what looks very much like a Switchcraft Les Paul socket, and has been soldered back into the board with fly leads.  Obviously this, being metal, has added an earth point that wasn't in the original design.

I'm off to see if I can find some nylon washers as a temporary measure, and at some point I'll put back a proper PCB mounted item.

It's still buzzing, obviously much more with single coils.  I'm starting to wonder if it's actually fine - they were inherently noisy by all accounts.  The clock frequency leaking into the audio path is annoying, but it's not like I'm recording with the Halle Orchestra.

On the upside, it has that lovely 80's modded-Marshall thing going on, and the clean sounds aren't too shabby either.  The chorus is a bit Marmite, I quite like it.  I cranked it last night with my old Floyd superstrat and had a big grin on my face for a good 45 minutes until Mrs. R. came in and I had to give it a rest .

ICBM

randella said:

@ICBM A bit more investigation, if you or your friend are interested.

He's got one already, and if I never see another one that will be quite OK .

It does sound like the input jack could be part of the ground-loop problem, yes.

Reverend

randella said:

Hi All

Dug out my old ADA last night for a blast, and remembered a) how much I like the tone, and b) just how noisy it is.  I'm not expecting it to be modern whisper silent but there's a lot of 50Hz hum present, plus a much more irritating highish-frequency whine with a rhythmic (maybe 2Hz) oscillation in volume all of which to my ears sounds like a clock signal.

The noise is present on all preamp settings, regardless of output level.  In place of a power amp, I'm using it in the effects return of a Mesa Mini Rectifier and controlling the overall volume with the MP-1's output level knob.

It's an ancient unit I bought for £70 in the late nineties, off a tech who claims it'd been on tour with a thrash metal band called Dearly Beheaded.  The story made me chuckle anyway, and when I got it home pretty much every single preset was a thick, heavy distortion with the odd crystal-clean-chorus patch for good measure so maybe it was true.  Who knows.

Either way, it ain't pristine.

Anyone any ideas as to what and what shouldn't be replaced?  It's been worked hard and I've no doubt several components will be past end of life.

I played with Dearly Beheaded once. Tim has killer band called Damnations Hammer now - classic Celtic Frost/Winter type band. 

randella

Reverend said:

I played with Dearly Beheaded once. Tim has killer band called Damnations Hammer now - classic Celtic Frost/Winter type band. 

@Reverend cool stuff - it's not really my bag but I might check it out this afternoon.  I'd bet you need some chops to play with guys like that.

I just always loved the name of the band.  I've no doubt that's where the ADA came from.  Just reading their FB bio, it seems their only tour was in '96 with Annihilator so the dates check out.  Apparently they shared a bill with Slayer on that tour, so my preamp's been keeping some mighty company!  Makes a good story if nothing else. 

randella

For anyone reading this for info in the future, isolating the input jack from the chassis made no difference. The whine I think is the display driver PWM frequency - with the lid off, the noise level fluctuates when the LED matrix flashes if you put the unit in edit mode.

I took it to rehearsal last night and the noise was OK. Our bassist is selling a load of old PA gear so I bought a noise gate off him for £25, probs going to leave it at that for now